Monoethylene glycol is used as a raw material in the manufacture of polyester fibres, polyethylene terephthalate (PET) plastics and resins. It is also incorporated into automobile antifreeze liquids.
Monoethylene glycol is typically prepared from ethylene oxide (EO), which is in turn prepared from ethylene. Ethylene and oxygen are passed over a silver-based catalyst, typically at pressures of 10-30 bar and temperatures of 200-300° C., producing a product stream typically comprising ethylene oxide, carbon dioxide, ethylene, oxygen and water. The amount of ethylene oxide in the product stream is usually between about 0.5 and 10 mole percent. The product stream is supplied to an ethylene oxide absorber and the ethylene oxide is absorbed by a recirculating solvent stream containing mostly water. The ethylene oxide-depleted stream is partially or entirely supplied to a carbon dioxide absorption column wherein the carbon dioxide is at least partially absorbed by a recirculating absorbent stream. Gases that are not absorbed by the recirculating absorbent stream are recombined with any gases bypassing the carbon dioxide absorption column and are recycled to the ethylene oxide reactor.
The solvent stream leaving the ethylene oxide absorber is referred to as fat absorbent. The fat absorbent is supplied to an ethylene oxide stripper, wherein ethylene oxide is removed from the fat absorbent as a vapour stream. The ethylene oxide-depleted solvent stream is referred to as lean absorbent and is recirculated to the ethylene oxide absorber to absorb further ethylene oxide.
The ethylene oxide obtained from the ethylene oxide stripper can be purified for storage and sale or can be further reacted to provide ethylene glycol. In one well-known process, ethylene oxide is reacted with a large excess of water in a non-catalytic process. This reaction typically produces a glycol product stream consisting of almost 90 weight percent monoethylene glycol, the remainder being predominantly diethylene glycol, some triethylene glycol and a small amount of higher homologues. In another well-known process, ethylene oxide is catalytically reacted with carbon dioxide to produce ethylene carbonate. The ethylene carbonate is subsequently hydrolysed to provide ethylene glycol. Reaction via ethylene carbonate significantly improves the selectivity of ethylene oxide conversion to monoethylene glycol.
Efforts have been made to simplify the process for obtaining ethylene glycol from ethylene, reducing the equipment that is required and reducing the energy consumption. GB 2 107 712 describes a process for preparing monoethylene glycol wherein the gases from the ethylene oxide reactor are supplied directly to a reactor wherein ethylene oxide is converted to ethylene carbonate or to a mixture of ethylene glycol and ethylene carbonate. EP 776 890 describes a process wherein the gases from the ethylene oxide reactor are supplied to an absorber wherein the absorbing solution mainly contains ethylene carbonate and ethylene glycol. The ethylene oxide in the absorbing solution is supplied to a carboxylation reactor and allowed to react with carbon dioxide in the presence of a carboxylation catalyst. The ethylene carbonate in the absorbing solution is subsequently supplied with the addition of water to a hydrolysis reactor and subjected to hydrolysis in the presence of a hydrolysis catalyst.
The present inventors have sought to further improve the manufacture of alkylene glycol from an alkene. In particular, the present inventors have sought to provide a process that reduces the cost and complexity of the plant whilst ensuring high selectivity.